For safety-related reasons, control systems governing the operation of motor-driven automotive panels (such as windows, sliding doors, trunk lids, sun-roofs, etc.) are typically designed to reverse the motor for retracting the panel in response to a detected engagement of the panel with an obstacle prior to reaching a desired position. See, for example, the U.S. Pat. No. 4,823,059, issued Apr. 18, 1989, which detects the presence of an obstacle in the path of a motor-driven deck lid when the motor current exceeds a predefined threshold. In order to avoid false detection of an obstacle, the control must somehow account for normally occurring variations in system voltage, wear and mechanical friction. For example, the control could be designed to measure the speed of the motor or panel, and to detect the presence of an obstacle if the measured speed is suddenly reduced in a range of panel displacement where an obstacle might be encountered. However, if the obstacle is relatively compliant, a sudden change in motor speed may not occur when the panel initially contacts the obstacle, and the control may fail to detect the obstacle as desired. If the detection thresholds are adjusted to increase sensitivity to changes in the measured speed, periodic undulations in the measured speed signal can be interpreted as a speed reduction, leading to false obstacle detection. Filtering may be used to remove the periodic undulation, but the signal delay caused by filtering correspondingly delays detection of the obstacle, which is undesirable.
Accordingly, what is needed is an improved obstacle detection method that is insensitive to variations in system voltage, wear and mechanical friction, and which is capable of reliably detecting the presence of an obstacle, even if the motor speed change due to the panel contacting the obstacle occurs only gradually.